The neem (or nim) tree is a subtropical tree which is native to the arid regions of India, Pakistan, Sri Lanka and parts of Southeast Asia and Western Africa. Once or twice a year, it bears a yellow, bitter fruit. The fruit contains a seed composed of a kernel and a husk. The kernel contains about 40 to 60% by weight neem oil. This oil can be isolated by standard procedures used in vegetable oil industry that involve expelling the oil in a cooker-prepress followed by extraction of the residual oil with hexane in a solvent extractor.
Neem oil has all the typical features of an edible, vegetable oil. It is composed of edible triglycerides anti non-edible or undesirable impurities such as phospholipids (gums), fatty acids, soaps, colored impurities, for example carotenoids and chlorophyll, and a host of other molecules. Such impurities are routinely removed in a typical refining process comprising the steps:
1. Degumming with water or an acid such as citric or phosphoric acid to remove phospholipids;. PA1 2. Caustic refining with sodium hydroxide to remove fatty acids and soaps. PA1 3. Absorption on beaching earth to remove colored materials. PA1 4. Steam distillation to remove low molecular weight materials.
The first two steps of the refining process are most commonly carried out on raw oils stripped of hexene. However, for some oils, such as cotton oil, it is preferable to refine hexane solutions of the oil, for example 65% oil in hexane. As part of the refining process, the oils are commonly hydrogenated in order to improve their thermal and storage stability.
In order to be considered an edible product, the oil has to pass certain criteria. Some of these are chemical tests while others such as taste and smell are more arbitrary and are dependent on local cultures and conditions. The chemical tests include analysis of the fatty acid content of triglycerides. Erucic acid (C.sub.20) and saturated acids such as palmitic (C.sub.6) and stearic (C.sub.18) are undesirable. Trace metals, chlorophyll, free fatty acids and phosphorous are also undesirable. Peroxide value. indicating stability towards oxidants, should be close to zero. Oils should be pale and their color is expressed by a standardized color index. In addition to these analyses. rape, mustard and canola oils are routinely assayed for sulfur which indicates the presence of glucosinolates. These compounds, occurring only in wild varieties, are catalyst poisons and therefore undesirable from the viewpoint of the refiners. More importantly, however, they lend the oil a specific odor and taste which are not popular with many consumers.
Analysis of crude neem oil suggests that it has the characteristics of a standard edible oil with regard to its fatty acid composition. Most of its impurities are of the standard, expected type except for sulfur, a very important exception. Two methods are in use for sulfur analysis the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP) analytical procedure and the Raney Nickel Reduction (Ra(Ni)) method. The ICP method is very rapid and convenient, but the industry standard in North America, &lt;1 part per million (ppm) for canola, is based on the older and rather cumbersome Ra(Ni) method which routinely gives values ten times lower than those obtained using the ICP method. The ICP and Ra(Ni) analytical procedures are described in "Sulfur Levels in Canola Oils from Canadian Crushing Plants Analysis by Raney Nickel Reduction and Inductively Coupled Plasma Atomic Emission Spectroscopy," Proceedings of the GCIRC Eighth International Rapeseed Congress, Jul. 9, 1991, Saskatoon, Sask., Canada, Vol. 5, page 1396, D. I. McGregor, ed., published by the Organizing Committee of the Eighth International Rapeseed Congress under the Auspices of the Groupe Consultatif International de Recherche sur le Colza (GCIRC) and the Canola Council of Canada, the disclosure of which is incorporated by reference herein. The sulfur content in neem oil is very high, 2200 ppm, using the ICP analytical procedure, and consequently imparts a very disagreeable odor and taste to the neem oil
All sulfur values reported in the specification and in the claims were obtained using the ICP method for sulfur analysis unless otherwise indicated.
Standard vegetable oil refining steps do not reduce the sulfur content down to an acceptable level in neem oil, although the other typical impurities are removed. Hydrogenation, to improve oil stability, was not possible due to the poisoning of the hydrogenation catalyst by the sulfur. This failure of the standard refining methodology may account for the lack of interest by food producers in neem oil.
C. Rukmini, Food Chemistry 26, 119-124 (1987) discloses a debitterized neem oil. However, the details for obtaining such debitterized neem oil are not revealed.